Paper making machine fabric for use in the drying sections of a paper making machine are sometimes composed of polyethylene terephthalate (PET) monofilaments which have previously been preferred over multifilament fabric because they clean easily, they have good abrasion resistance, and they leave a more desired surface on the paper. The life of hydrolyrically stabilized polyethylene terephthalate fabrics is still limited mainly because of hydrolysis of the polyethylene terephthalate ester linkages. Extended life of the monofilament fabrics is desired in order to reduce paper machine downtime. Increased paper making machine speed can be achieved with these new fabrics if the same lifetime is planned. In addition hydrolyrically stabilized polyethylene terephthalate monofilament fabrics do not have sufficient life to be used in certain brown paper and higher speed paper making operations. Under these more severe drying conditions fabrics composed of multifilament yarns of NOMEX (an aromatic polyamide fiber sold by DuPont), glass, and PET are used. It is desired to replace these multifilament yarn fabrics with monofilament fabrics for ease of cleaning and better paper surface characteristics. No monofilament fabrics with acceptable cost/performance levels have been found for these more severe drying applications. Monofilament fabrics which are used in food contact processing usually require U.S. Food and Drug Administration approval. Most PET hydrolytic stabilizers used in the monofilament fabrics do not have FDA approval. Because of this lack of FDA approval, PET monofilament fabrics are used in food processing without hydrolytic stabilizers which results in a short life span for these fabrics.
Polypropylene is the cheapest material presently available; it has excellent hydrolytic stability, but poor dimensional stability at elevated temperature, and as a result it has only limited use.
Polyphenylene sulfide has excellent dimensional and hydrolytic stability, but suffers from the disadvantage that it is extremely highly priced, is more difficult to work, and tends to suffer from brittle fracture problems in the crystalline state due to normal flexing experienced on the paper machine.
U.S. Pat. No. 4,026,973, assigned to Teijin Limited, Osaka, Japan, discloses heat resistant polyester filaments comprising an aromatic polyester in which at least 90 mole % of the structural units are ethylene-2,6-naphthalate units having specific properties. Copolymers reported to be useful include those containing less than 10 mole % and preferably less than 5 mole % comonomer. A wide range of modifying acids and glycols are listed. This list includes 2,7-naphthalenedicarboxylic acid.
U.S. Pat. No. 3,161,710 discloses a polymerization process for polyester films. A large number of polyester homopolymers and copolyesters are disclosed including those containing 0 to 25 mole % of 2,6-, and 2,7-naphthalenedicarboxylic acid.
Some patents dealing with PEN as fibers for use in applications such as tire cord include Japan Kokai 156,312 (1987), U.S. Pat. No. 3,616,832 (1969), Great Britain 1,445,464 (1976), and U.S. Pat. No. 4,000,239 (1976).
U.S. Pat. No. 3,975,329 (1976) assigned to The Goodyear Tire and Rubber Co. describes industrial polyester yarns such as those based on poly(ethylene terephthalate) having improved hydrolytic and thermal stability. The improved stability is reported to be achieved by spinning the polyester in the presence of a carbodiimide which is reported to react with free carboxyl groups.
U.S. Pat. No. 4,829,681 (1989) assigned to Albany International Corp. discloses a paper machine dryer fabric which comprises fabrics made from interwoven synthetic, polymeric polyester resins.
U.S. Pat. No. 5,169,499 (1992) assigned to Albany International Corp. discloses an article of paper machine clothing which consists essentially of a woven polyester material which is a copolyester of terephthalic acid, isophthalic acid and 1,4-cyclohexanedimethanol.
None of the above described references solve all of the numerous problems in the art of lack of resistance to hydrolytic and dry heat degradation, lack of dimensional thermal stability, and lack of resistance to deformation.
Therefore, there is a need in the art for an article of paper machine fabric containing a fibrous structure with improved resistance to hydrolytic and dry heat degradation, improved dimensional thermal stability, and improved resistance to deformation which overcomes the above described deficiencies.